1. Field of Invention
This invention relates to two-dimensional positioning apparatus, and more particularly, to such apparatus wherein components thereof and methods used therein are improved.
2. Description of the Prior Art
A two-dimensional positioning apparatus is disclosed, for example, in Japan unexamined patent application No. 2000-65970, and shown in FIG. 1, wherein a platen 10, which is made of a magnetic material, is provided with teeth formed at fixed spacings in the X-axis and Y-axis directions. Only part of the teeth are shown for simplicity of description. An object to be positioned is placed on slider 11. Levitating means 12 causes slider 11 to be disposed or levitated above platen 10. Nozzles are provided on the surface of slider 11 facing platen 10 Jets of compressed air are directed through the nozzles by levitating means 12 to produce a levitating force.
A Y-axis motor 13 is mounted on slider 11 and teeth 132 are formed on Y-axis motor 13 at fixed spacings in the Y-axis direction. Y-axis motor produces magnetic attractive force between teeth 132 and teeth 101 of platen 10 to cause slider 11 to move in the Y-axis direction.
X-axis motors 14 and 15 are mounted on slider 11 to be symmetrically opposite to each other in relation to the center point of slider 11. Teeth 141 and 151 are formed on X-axis motors 14 and 15 at fixed spacings in the X-axis direction. X-axis motors 14 and 15 provide magnetic attractive force between teeth 141 and 101 and between teeth 151 and 101 to cause slider 11 to move in the X-axis direction. Connecting members 111 and 112 connect X-axis motor 13 to both X-axis motors 14 and 15.
An X-axis mirror 16 is attached to one side of platen 10, and a mirror surface is formed in the Y-axis direction. A Y-axis mirror 17 is attached to another side adjacent to the side of platen 10, and a mirror surface is formed in the X-axis direction.
A Y-axis position sensor 18, which is mounted on Y-axis motor 13, is a laser interferometer that emits light beams to imping Y-axis mirror 17, receives catoptric light beams from Y-axis mirror 17, and detects Y-axis position of slider 11 by means of optical interference.
X-axis position sensors 19 and 20, which are mounted on the X-axis motors 14 and 15, respectively, are laser interferometers that emit light beams to X-axis mirror 16, receive catoptric light beams from X-axis mirror 16, and detect X-axis position of slider 11 by means of optical interference.
A Y-axis controller 21 feedback controls the position of slider 11 according to the deviation of a Y-axis directive position from a position detected by Y-axis position sensor 18.
X-axis controllers 22 and 23 feedback control the position of slider 11 according to deviations of X-axis directive positions from positions detected by X-axis position sensors 19 and 20.
A rotational error may occur around an axis perpendicular to the X and Y axes of slider 11. This phenomenon is referred to as yawing and the angle of rotational error, i.e. the yaw angle, is assumed to be xcex8.
In the apparatus shown in FIG. 1, the X-axis and xcex8-axis positions are controlled by supplying the same position command to X-axis controllers 22 and 23. The state in which any yawing in slider 11 is eliminated is defined as xcex8=0.
For light beams emitted by Y-axis position sensor 18 and X-axis position sensors 19 and 20 toward mirrors to be able to correctly return to their respective sensors, the yaw angle must be maintained at nearly zero, i.e. xcex8=0. If the yaw angle xcex8 deflects the light in a large measure, the light beams emitted by Y-axis position sensor 18 and X-axis positions sensors 19 and 20 will fail to return to the sensors. Thus, the position of slider 11 will be unknown, and hence, the position and speed of slider 11 cannot be feedback controlled. Since the position sensors are optical sensors using laser interferometers, even a small rotational error of slider 11 can result in lack of control.
In the FIG. 1 apparatus, it is difficult to adjust angle xcex8 to be close to 0 for the following reasons: First, it is not possible to separately set the control characteristics of the xcex8-axis and X-axis directions. To be able to effect control and satisfy the angle xcex8=0, the servomechanical rigidity of angle xcex8 may be increased. However, the servomechanical ridigity in the xcex8-axis direction is uniquely fixed when the control methods and bandwidths of the X-axis controllers 22 and 23 are fixed. Second, control in the xcex8-axis direction becomes difficult or impossible when acceleration in the X-axis direction is at its maximum.
The output torque T of slider 11 is represented by the following equation:
T=Fx2xc2x7Lx2xe2x88x92Fx1xc2x7Lx1
wherein, Fx1 is the propulsion force of X-axis motor 14; Fx2 is the propulsion force of X-axis motor 15; Lx1 is the Y-axis distance from the center of gravity of slider 11 to the center point of X-axis motor 4; and Lx2 is the Y-axis distance from the center point of X-axis motor 15 to the center of gravity of slider 11.
If the load on slider 11 is large and the value of an acceleration/deceleration command signal for the X-axis direction is also large, the propulsion force Fx1 and Fx2 of X-axis motor 14 and 15 are at a maximum. Assuming the maximum values of Fx1 and Fx2 are Fx1max and Fx2max, then the output torque T of slider 11 is
T=Fx2maxxc2x7Lx2xe2x88x92Fx1maxxc2x7Lx1.
If Fx1maxxc2x7Lx1xe2x89xa0Fx2maxxc2x7Lx2 holds true for reasons of manufacturing variations, for example, then angle xcex8 will also increase. Even when Fx1maxxc2x7Lx1=Fx2maxxc2x7Lx2 is true, angle xcex8 will also increase and servo control becomes difficult if not impossible when a disturbing torque Td is applied.
As discussed, in the FIG. 1 apparatus, propulsion force is consumed only for control in the X-axis-direction and no consideration is provided for consuming propulsion force for control in the xcex8-axis direction. This approach results in an unbalanced maximum propulsion force being applied to the two X-axis motors, or angle of yaw xcex8 increasing when, for example, a disturbing torque interferes. Hence, servo control is difficult if not impossible.
Accordingly, an object of the invention is to overcome the aforementioned and other problems, disadvantages and deficiencies of the prior art.
The foregoing and other objects are attained by the invention which encompasses a two-dimensional positioning apparatus that provides for position control even when rotational errors occur in the slider of the apparatus by performing control separately in the X-axis and xcex8-axis directions.
In other aspects of the invention, an interferometer is used with angular frequency being modulated according to amount of movement by an object and multiplied by a reference signal so that a high frequency signal is provided even when the slider is stopped or moved at a low speed; and a motor drive circuit is provided having a feedback control loop that employes a compensation for signals near a zero crossing point so that a deadband near the zero crossing is eliminated; and a return to origin slip plate having two slits arranged in the Y-axis and X-axis directions is provided to detect the position of the slider with the two sliders detecting an interference of laser light, so that a change in wavelength of the laser light, such as due to aging, is detected.